The invention involves a process for the double-sided printing and/or coating of a substrate, in particular, of paper or cardboard, using at least one toner.
A known process is electrostatic printing, in which a latent electrostatic image is developed by charged toner particles. These particles are transferred onto an image-receiving substrate, called substrate for short in the following. Afterwards, the developed image that has been transferred onto the substrate is fixed by the toner particles being heated up and fused. To fuse the toner particles, contacting processes are often used in which the toner particles are brought into contact with suitable devices, for example hot rollers or cylinders. It is disadvantageous that it is usually necessary to use silicone oil as a separating agent that should prevent an adhesion of the fused toner onto the heating device. In addition, the design, the maintenance and the operating costs of these heating devices that operate by contact are expensive and thus cost-intensive. Furthermore, the defect rate caused by the contacting heating devices is relatively high. In order to fix the toner that is transferred onto the paper, for example, heating devices and processes are also known that operate in a contactless manner, in which for example, the toner particles are fused using heat radiation and microwave radiation or with hot air.
In the contacting and non-contacting fusing processes, toner is used, for example, that has a glass transformation temperature (TG) in a range from 45xc2x0 C. to 75xc2x0 C. The glass transformation temperature, at which the tonerxe2x80x94starting from the solid statexe2x80x94begins to soften, can be influenced by the choice of raw materials and by the addition of certain additives to the toner. The lower value of the temperature range, in which the glass transformation point lies, is limited at the bottom by the storage conditions of the toner and the heat generated in the printer, in particular within the development station, and it is limited at the top by the fusing and fixing conditions. In a fusing device for the toner, both the toner as well as the substrate itself is heated up. In order to be able to ensure a good fixing of the toner onto the substrate, the surface temperature of the substrate must be in the range of the glass transformation temperature of the toner above it. The toner easily reaches and/or exceeds the glass transformation temperature (TG) in the area of the fusing device.
Processes and devices are known in which the substrate is printed or coated double-sided, whereby for the printing of the upper side and the lower side, one and the same toner transfer device and fusing device is often used. After a first side of the substrate is printed, the substrate is automatically reversed, supplied back to the beginning of the processing line, and supplied again to the transfer device and fusing device, where the other side of the substrate is printed. While the toner located on the second side of the substrate is fused, the substrate, the image that has already been fixed on the rear side of the substrate, and the image to be fixed are heated. The second heating affects the print quality in an undesirable way, in particular the gloss of the image that has already been fixed and is located on the first side. By the repeated heating of the substrate, the gloss can change at individual locations or over the entire side of the substrate. The gloss value of the second side of the substrate is larger than on the first side of the substrate. Furthermore, the toner already fixed on the first side of the substrate tends to smudge when the second side of the substrate is heated up to a temperature that is above the glass transformation point of the toner. The renewed fusing of the toner that has already been fixed and is located on the first side of the substrate leads to errors in the printed image and to the smudged toner dirtying a transport device that conducts the substrate along the processing line. In the worst case, the substrate can adhere to the transport device. The same problems also occur in a device in which two complete print units each have a toner transfer device and a fusing device. In these known devices, a first image is transferred and fixed by a first print unit to a first side of the substrate, while subsequently a second image is transferred and fixed onto the rear side of the substrate using the second print unit.
The purpose of the invention is to produce a process in which a double-sided printing and/or coating of a substrate is possible with a simultaneously high quality of the images and/or coatings applied onto the front side and the rear side of the substrate.
In order to achieve this purpose, a process is proposed that provides for the double-sided printing and/or coating of a substrate, for example, a paper sheet or a paper web, while using at least one liquid or dry toner that has at least one polymer, at first at least one toner layer, or a first image that has at least one toner layer, is transferred onto a first side of the substrate. Then, this toner is heated up to its glass transformation temperature (TG) or a temperature above it. In the process, the toner and/or the toner layers are preferably fused until a certain gloss becomes set. This state of the toner is then transformed by the fixing of the toner onto the substrate using ultraviolet radiation, for example. The toner present in the form of individual molecules has the property that its original glass transformation temperature shifts to a higher temperature level as a result of the cross-linking of its polymer chains, and the viscosity of the toner increases. In other words, after the toner has been heated up for the first time to its glass transformation point, or beyond, and cross-linked, and cooled off again, its glass transformation temperature increases so that this toner first softens at a higher temperature when it is re-heatedxe2x80x94starting from the solid state. The cross-linking process increases the glass transformation temperature and the viscosity of the toner, so that the toner no longer becomes liquid above its new glass transformation temperature when it is re-heated, but instead it obtains a thermoplastic, rubber-like structure. After the toner has then been fixed to the first side of the substrate, at least one toner layer or a second image that has at least one toner layer is transferred to the other, second side of the substrate in the next step. The toner located on the second side of the substrate is then warmed or heated up to a temperature that is equal to or greater than its own glass transformation temperature. Next, a cross-linking of the molecules of the second toner also occurs here, which leads to the changes in the properties of the toner as described above. Since the toner already fixed onto the first side of the substrate can no longer become liquid (as mentioned), but stays highly viscous when heated above its new glass point, it can be ensured that the toner applied and fixed onto the first side of the substrate does not smudge on its support, for example, a conveyor belt or a roller, or experience a change in its gloss, by the fixing of the toner on the other, second side of the substrate.
It is especially advantageous in the process according to the invention that the temperature of the first side of the substrate and the toner fixed on it, which becomes set during the heating of the second side of the substrate in order to fix the second toner image, can also be above the new glass transformation point of the first toner image, provided the first toner image is not harmed by this. By the toner located on the first side of the substrate no longer becoming liquid when it is re-heated, it is thus possible to prevent a smudging and thus a dirtying of the printing and/or coating machine and/or copier, in which the process according to the invention is applied, by the toner applied and fixed on the first side of the substrate. It is furthermore advantageous that the quality, in particular, the gloss of the image and/or the coating applied onto the first side of the substrate remains the same and does not change when the second side of the substrate is printed or coated.
In a preferred embodiment form, the glass transformation temperature of the toner increases, because of the cross-linking of the polymer chains, by 10xc2x0 C. to 20xc2x0 C. and at the same time, the viscosity of the toner increases. Above the glass transformation point, the toner is no longer liquid when it is re-heated, but instead (as mentioned) obtains a thermoplastic, rubber-like structure. This and other effects cause the gloss of the first image and/or the coating on the first side of the substrate to no longer change during printing and/or coating of the second side of the substrate.
In a preferred embodiment form, a powdery dry toner is used that has a glass transformation temperature preferably in a range from 45xc2x0 C. to 75xc2x0 C. and a glass transformation point that shifts by approx. 10xc2x0 C. to 20xc2x0 C. after it is heated up for the first time above its original glass transformation temperature with subsequent cross-linking of the toner, so that the lower value of its new glass transformation temperature is in the range from 55xc2x0 C. to 65xc2x0 C. or higher. Especially preferred is a dry toner that is cross-linked by, and preferably exclusively by, irradiation with ultraviolet light, that has a glass transformation point above 45xc2x0 C. prior to being fused for the first time and is comprised of the following components:
1. Uralac XP 3125 (polyester resin) with approx. 83 percent by weight ([symbol] 79.05% portion of total weight of the toner)
2. Uralac ZW 3307 (cross-linking agent) with approx. 17 percent by weight ([symbol] 16.19% portion of total weight of the toner)
3. Irgacure 184 (photo initiator) with approx. 1 percent by weight ([symbol] 0.95% portion of total weight of the toner) and
4. BASF Heliogon Blue 7090 (color pigment) with approx. 4 percent by weight ([symbol] 3.81% portion of total weight of the toner)
Optionally, additives to control the melt flow, the surface quality, the toner charge, the powder flow, and if necessary, additional additives are also added to the mix.
The raw materials of this toner are mixed together and molten-mixed in a heated two-roller mill, for example. The cooled-off extrudate is milled to a particle size xe2x89xa73 mm and then brought into a fluid-energy mill which pulverizes it further. Finally, the fine toner particles are sorted, whereby for the toner used in the process according to the invention, preferably particles having an average particle size of approx. 8 xcexcm are used. The fusing of the toner for the purpose of fixing it onto its substrate is done at a surface temperature of approx. 70xc2x0 C. to 120xc2x0 C., at which the curing of the toner is also performed as a result of the cross-linking of the polymer chains when the fused toner is irradiated with ultraviolet light. By the cross-linking of the polymer chains, the glass transformation temperature of the toner increases by over 10xc2x0 C., and its viscosity also increases. With regard to the composition of the toner, the realizable fusing process, and the fixing process, reference is made to the publication xe2x80x9cUV-cured Toners for Printing and Coating on Paper-like Substancesxe2x80x9d by Detlef Schulze-Hagenest and Paul H. G. Binda, ISandT 13th Int. Congr. Adv. i. Non-Impact-Printing Technologies, 1997, the content of which has been made an object of this application.
Provided the substrate is paper, cardboard, or the like, its first side can be the front side and its second side can be the rear side. Of course, it is also possible that the first side of the substrate is the rear side and the second side of the substrate is the front side of the paper. In other words, whether the front side or the rear side of the paper is printed first can be freely chosen.
In a preferred embodiment form of the process, it is proposed that the fixing of the toner is done in a contactless manner. For this purpose, for example, a known drying oven, heat radiation and/or microwave radiation and/or hot air or the like can be used. Especially preferred is an embodiment variation in which the toner is fixed exclusively with ultraviolet radiation, i.e. is cross-linked in the fused state. The fusing of the toner can, for example, be done using or exclusively by infrared radiation, hot air, microwaves and/or the like.
Furthermore, an embodiment form of the process is preferred which is characterized in that several toners with different colors are applied onto at least one of the sides of the substrate. The image applied on one side of the substrate thus has several colors, for example, black, cyan, magenta, yellow, and/or a secondary color. With the process according to the invention, not only is a single-color print readily realizable, but also a multi-color print, whereby here it also applies that the glass transformation point of each of the toners increases after the toner is heated and fixed for the first time, for example, by up to 10xc2x0 C. or more. Furthermore, the properties of the toner change, which, upon renewed heating to its now new glass transformation temperature or above it, no longer becomes liquid, but instead obtains a thermoplastic, rubber-like structure. In this way, it is ensured that during printing or coating of the second side of the substrate, the toner already fixed to the first side of the substrate does not become liquid again.
In a preferred embodiment form, up to seven toners with different colors can be transferred and fixed in order to generate the image or a coating on the substrate. Preferably, however, only four different toners with different colors, for example, the primary colors, can be applied. It is to be emphasized that in relation to the invention presented here, the term xe2x80x9ccoatingxe2x80x9d is understood to be a thin layer formed from at least one toner. A xe2x80x9ccoatingxe2x80x9d can thus also easily have several different-colored toners so that the coating can also be multi-colored.
Furthermore, an embodiment form of the process is preferred which is characterized in that the toners at first are all applied onto the respective side of the substrate in order to generate a coating or an image and then heated together and fixed. Thus, on each of the two sides of the substrate, respectively, only one fixing operation is performed. In another embodiment example, it is planned that on at least one of the sides of the substrate, several fixing operations are performed in order to generate the image or coating. For example, after each transfer of a toner layer onto a side of the substrate, it can then be fixed immediately onto the substrate, whereby then in a subsequent step, the next toner layer is applied onto the substrate, which in turn is fixed immediately after that. Of course, for example, at first two toner layers can also be applied onto a side of the substrate which then are fused together and fixed, whereby in a subsequent process step on the same side of the substrate, an additional toner layer is transferred onto the toner layers that have already been fixed, and this toner layer is then bonded with the substrate in a subsequent separate fixing operation.
In a preferred embodiment form, the process according to the invention can be used in conjunction with a digital printing machine, i.e. a machine that operates, for example, according to the electrographic or electrophotographic process. The process can be applied fundamentally anywhere that using at least one toner, a substrate is coated or an image is transferred to a substrate and fixed there. The printing machine can thus also be a copier.
Additional advantageous embodiment forms of the process result from the remaining subordinate claims.